Bat Echolocation Researc h - Bat Conservation International

Section 2: Acoustic Inventories
Figure 10: A mobile survey with a focus on what is perceived as ‘good’ bat habitat will lead to an assessment
which is different from stationary set ups. The mobile approach will lead to a good cover of the batoccurrence
landscape use in an area (A, C), whereas the stationary set up can provide detailed, quantitative
data regarding the sample point (B, D). The observed spectrum of species might differ, where the
mobile approach provides the opportunity to investigate particular habitats for target species.
(Table 3, Fig 8). This summary
can serve as a tool for selecting
appropriate detectors. As examples
at different ends of the spectrum
of typical applications, questions
that concern biogeography
or the presence of a species in different
habitats (upper left corner,
Table 3, Fig. 8), require discriminative
parameters of calls, but it is
not necessary to identify all calls
or to necessarily have the best
characterization possible of call
features. If large areas are to be
surveyed, high mobility and several
observers are needed, suggesting
the need for multiple
detectors of relatively low cost. In
this situation, a successful
approach could involve heterodyne
detectors supported by
some sets of TE or FD detectors
for recording and back-up analysis
(Fig. 9; Limpens et al. 1997).
In contrast, questions about ecology and differences in
echolocation calls within the context of how species fit
into feeding guilds (upper right corner, Table 3, Fig. 8)
demand high-quality recordings with the best acoustic
information possible. Between the ‘extremes’ in the
upper left and upper right corners of Table 3, different
research approaches are possible. These approaches
include surveys or transects with heterodyne detectors
and combinations of heterodyne with FD or TE for
back-up analysis (Limpens 1993; Walsh and Catto 1999;
Walsh and Harris 1996a, 1996b), to setups with arrays of
stationary systems involving TE, FD, or heterodyne
detectors generating automated recordings on DAT or
computer (Britzke et al. 1999; Jones et al. 2000).
Where habitat use is the focus, a number of stationary
detectors (i.e., sample points) may be the most
appropriate. It is worth noting that an expensive detector
at a fixed position will not monitor all habitats
around the sample position equivalently, and sampling
bias could lead to a different spectrum of species
observed. Nor will the detector document differences in
echolocation behavior in relation to flight patterns within
various habitat features around the sampling point. In
such cases, the use of stationary detectors on fixed sample
points coupled with observers with the capability of
TE recordings, would be complementary.
Recent work demonstrates that studies of habitat use
must take into consideration that bats use different strata,
ranging from near the ground, to thousands of meters
above the ground (Fenton and Griffin 1997; Griffin and
Thompson 1982; McCracken 1996; Menzel et al. 2000).
Because of the remote nature of altitudinal surveys, visual
observation to enhance identification and tuning of
the heterodyne system are difficult or impossible. Therefore,
the use of broadband detectors is advisable.
Although the detection range is limited, frequency-division
systems seem most applicable to remote monitoring
of bat activity because they are continuously monitoring
the landscape. Where species identification is required,
time-expansion systems may be preferred because they
provide maximum information. In a time-expansion/heterodyne
combination, the latter system continuously
monitors the landscape, thus compensating for the timesampling
and non-real-time performance required of the
time-expansion system.
In situations where bat activity is within, above, or
near structures, such as roads, tunnels, wind power utility
structures, caves, etc., relatively simple stationary
automated setups with heterodyne detectors and signalactivated
recorders can be effective, with the use of time
expansion for back-up recording of high quality signals
(e.g. Bach and Burkhardt 2000; Bach et al. 1999, 2001;
Rahmel et al. 1999).
Long-term observations are needed to document
changes in activity patterns and trends over the course
of seasons or years (Table 3, Fig. 8). Such studies might
concentrate on where stationary setups are feasible.
Where activity patterns and trends in populations in
larger areas are involved, larger numbers of sample
points, transects, or observers are needed. These characteristics
require low profile and low cost approaches,
where an accurate assessment of relative bat density
rather than high-tech processing of sound is needed.
Here again, heterodyne detectors, supported by some
sets of time-expansion (or frequency-division) detectors
for recording and back-up analysis can be a successful
approach (Walsh and Harris 1996a, 1996b; Walsh and
Catto 1999; de Wijs 1999).
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